In the field of advanced hard disk drives which are increased in recording density, it is desired to have a resist material capable of forming an isolated micropattern having a film thickness of 3 to 6 μm and a line width spacing of equal to or less than 0.5 μm. In the prior art, novolak base resist materials are prepared using two main components, a novolak resin and a photosensitive agent. The resist materials which meet the above requirement should have a higher sensitivity and resolution, improved pattern shape, heat resistance, film retention, substrate adhesion and storage stability as well as being adaptable to the wavelength of the light source in any exposure device. To this end, a number of designs have been proposed for the novolak resin, photosensitive agent and solvent.
Referring to the novolak resin, it is known from BREAK THROUGH, February 1992, page 18 that as the average molecular weight of a novolak resin being synthesized is reduced, the resolution is improved, but the heat resistance and film retention become poorer. It was then proposed to treat the novolak resin by a reprecipitation method for removing lower molecular weight fractions from the novolak resin. Also, JP-A 2002-296772 discloses a positive resist composition comprising a novolak resin in which some phenolic hydroxyl groups are substituted with 1,2-naphthoquinonediazidosulfonyl ester groups, a 1,2-naphthoquinonediazidosulfonyl ester of a polynuclear phenolic monomer, and a sensitizer. This positive resist composition is fully effective for forming a whole pattern at a film thickness of up to 1 μm, but lacks a high sensitivity and revolution sufficient to form an isolated pattern of thick film having a line width spacing of up to 0.5 μm.
With respect to the sensitizer, a positive resist composition comprising an alkali soluble resin, a quinonediazido group-containing compound, and a certain hydroxy compound was proposed in JP-A 9-319078 for achieving a high sensitivity and resolution and improving the resistance to dry etching necessary for semiconductor device manufacturing process. This positive resist composition is fully effective for forming a line-and-space pattern at a film thickness of approximately 1 μm, but lacks a high revolution sufficient to form an isolated pattern of thick film having a line width spacing of up to 0.5 μm.